The renin-angiotensin system (RAS) is a hormonal cascade of major critical importance to the regulation of blood pressure, fluid and electrolyte balance and kidney function. Angiotensin II (Ang II), the main effector peptide of the RAS, acts at two major receptors, AT1 and AT2. The vast majority of Ang II actions are mediated by the AT1 receptor, including cell proliferation, vasoconstriction and antinatriuresis. Much less is known concerning the functions of the AT2 receptor. Recent studies indicate that the AT2 receptor inhibits cell growth and induces vasodilation opposing the effects of Ang II at AT1 receptors. The role of the AT2 receptors in fluid and electrolyte homeostasis is unknown. The Principal Investigator has pilot data demonstrating that the AT2 receptor mediates natriuresis and that the haptapeptide derivative of Ang II, des-aspartyl-Ang II (Ang III), is the preferred agonist. The Principal Investigator also has preliminary data strongly suggesting that dopamine D1-receptor stimulation induces increased renal proximal tubule cell apical membrane AT2 receptor expression and that inhibition of AT2 receptors abolishes D1-receptor-induced natriuresis. This project will explore in depth the site and mechanisms of AT2 receptor-induced natriuresis. The project will focus on two specific hypotheses: (1) the AT2 receptor is a key mediator of the natriuretic response to Ang III, D1 receptor activation and increased renal perfusion pressure;and (2) natriuretic responses to Ang II and D1 like receptor activation require AT2 receptor recruitment to the apical plasma membrane of renal proximal tubule cells and are mediated by bradykinin, nitric oxide and cyclic GMP. The project will apply a combination of state-of-the-art in vivo and cell and molecular techniques to clarify the role of the AT2 receptor in sodium excretion. These studies will help clarify the pathophysiology of primary hypertension, a disorder affecting one-quarter after adult population in the Western world. PUBLIC HEALTH RELEVANCE Hypertension (high blood pressure), present in over 25% of the population of the Western world, is a major risk factor for heart and blood vessel disease leading to premature death and disability. Retention of salt and water by the kidney is required for hypertension to develop. This application will increase our understanding of the mechanisms whereby the kidney renin-angiotensin system regulates salt and water excretion, suggesting new molecular targets for the treatment and prevention of hypertension.